Alright, so
1. Skins would need some time to include, but it's a good idea and would take less space in the roster
2.Character-specific invincibility themes would be great, thanks for the suggestion
3.Mega Man already has assets being made, and Knuckles, Tails, Amy, Shadow & Rouge are an obligatory inclusion.
And 4. Tetromino is an experimental character and will be fixed in the coming updates, either demoted to a skin or something a bit less broken.
I appreciate the feedback!